I’m currently using an Iridium plug, but I have to say it makes no discernible difference to starting, running, performance or economy over a standard plug (not that I expected it to). The only advantage of an iridium plug is the iridium alloy makes for a very hard centre electrode, it also has a very high melting point, which makes them last longer. Because they are harder wearing the centre electrode can be made smaller, which theoretically allows for a slightly lower voltage to create an arc and the arc tends to follow the same track more consistently. On the down side, iridium is very brittle (so the gap is pre-set and difficult to alter without risking damage) and a lot more expensive. They were originally designed to enable the massively long service intervals we have on modern cars.

Which you choose to use is personal choice, just don’t expect any improvement in performance. Most people who marvel at the new-found performance gains after fitting iridium plugs are just noticing the effects of a new plug, or rather how worn out their old plug was! Any clean and fresh, good quality, correctly gapped plug will do its job perfectly well on an XT660. Personally I always prefer to use NGK, over Champion, Bosch and Denso etc. Everyone has their favourites though!

However, what you do need to watch out for are fake NGK iridium plugs if buying on ebay. There are quite a lot in circulation and can cause running problems at best, and at worst insulator or centre/ground electrode breakage and broken bits of ceramic and metal in your combustion chamber – not good. Standard plugs aren’t often counterfeited because of their lower value, so you’re less likely to pick up a duff one. To spot a fake read this.

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